1. Field of the Invention
The present invention relates to an imaging lens and, in particular, to an imaging lens applicable to a wide range of uses such as a taking lens for a video camera or still video camera used for TV phone, door phone, monitoring purposes, or the like, an imaging lens mounted in a scanner for capturing images for personal computers, and so forth.
2. Description of the Prior Art
Recently, solid-state imaging devices have often been disposed on imaging surfaces of various kinds of video cameras and still video cameras. With the advance of technology, such solid-state imaging devices have been reducing their size year by year, while requiring imaging lenses with smaller size. Also, there is an increasing demand for those with lower manufacturing cost.
Meanwhile, since the imaging lenses used for video camera or the like for TV phone, door phone, monitoring purposes, and so forth are required to attain compactness and lower cost in particular as mentioned above, many of them have been known to employ plastics as their lens-forming material.
When a plastic lens is employed, however, its focus greatly fluctuates upon changes in temperature in general, thus making it necessary to suppress such focus fluctuation. In order to restrain focus from fluctuating upon changes in temperature, a larger number of lens sheets must be employed, thereby increasing the size of the lens or its manufacturing cost. Therefore, though the necessity for taking account of temperature compensation in plastic lenses is recognized, there are many imaging lenses providing no means therefor.
Focus fluctuation upon changes in temperature can be corrected by refocusing, increasing Fno, restricting the usable temperature range, and so forth. Such means, however, are hard to be considered realistic. Thus, there has been a demand for developing a better technique therefor.